1. Field of the Invention
The present invention relates to a control apparatus and method and a control unit which modulate a value calculated by a feedback control method such that a controlled variable is caused to converge to a target controlled variable, with a predetermined modulation algorithm, to thereby calculate a control input to a controlled object using the modulated value.
2. Description of the Related Art
As a control apparatus for controlling the air-fuel ratio of a mixture supplied to an internal combustion engine, the present assignee has already proposed a control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550. This control apparatus is comprised of a LAF sensor, an oxygen concentration sensor, a state predictor, an onboard identifier, a sliding mode controller, and a target air-fuel ratio-calculating section. The LAF sensor and the oxygen concentration sensor are each for detecting a value indicative of the concentration of oxygen in exhaust gases flowing through an exhaust passage of the engine, i.e. the air-fuel ratio, and are inserted into the exhaust passage at respective locations downstream of a collecting section thereof. Further, the LAF sensor is disposed on the upstream side of a catalytic device, and the oxygen concentration sensor is disposed on the downstream side of the catalytic device.
This control apparatus employs a discrete-time system model as a controlled object model to which is input the difference DKACT between an actual air-fuel ratio KACT detected by the LAF sensor and a learned value FLAFBASE (hereinafter referred to as “the air-fuel ratio difference DKACT”) and from which is output the difference DVO2 between an output VOUT from the oxygen concentration sensor and a predetermined target value VOUT_TARGET (hereinafter referred to as “the output difference DVO2”), and calculates a target air-fuel ratio KCMD (target equivalent ratio) as a control input, as described hereinafter.
More specifically, the state predictor calculates a predicted value of the output difference DVO2 with a predetermined prediction algorithm based on the above-described controlled object model, and the onboard identifier identifies a model parameter of the controlled object model by an sequential least-squares method. Further, the sliding mode controller calculates an operation amount Usl based on the predicted value of the output difference and an identified value of the model parameter with a sliding mode control algorithm such that the output difference DVO2 converges to 0.
Furthermore, the target air-fuel ratio-calculating section calculates the learned value FLAFBASE by adding a learning component flafadp to a fixed value flafbase. When Uadp, which is a component of the operation amount Usl, is within a predetermined range, and KACT≈KCMD holds, the learning component flafadp is held at an immediately preceding value thereof, whereas when Uadp is not within the predetermined range, the learning component flafadp is calculated by adding or subtracting a predetermined value Δflaf to or from the immediately preceding value thereof. A calculation period or a learning speed at which the target air-fuel ratio-calculating section calculates or learns the learned value FLAFBASE is set to a relatively long period or low speed so as to prevent the calculation of the learned value from interfering with sliding mode control by sliding mode controllers.
Then, the target air-fuel ratio KCMD is calculated by adding the operation amount Usl to the learned value FLAFBASE calculated as above. As a result, a fuel injection amount is calculated such that the actual air-fuel ratio KACT converges to the target air-fuel ratio KCMD, whereby the air-fuel ratio is controlled such that the output VOUT from the oxygen concentration sensor converges to a predetermined target value VOUT_TARGET. The predetermined target value VOUT_TARGET is set to such a value as will make it possible to obtain an excellent exhaust emission reduction rate of the catalytic device when the output VOUT from the oxygen concentration sensor takes the target value VOUT_TARGET. As a consequence, it is possible to obtain an excellent exhaust emission reduction rate of the catalytic device by the above control.
Further, the present assignee has already proposed a control apparatus which controls a controlled object having a nonlinear characteristic in Japanese Laid-Open Patent Publication (Kokai) No. 2005-275489. In the control apparatus disclosed in FIG. 9 of Japanese Laid-Open Patent Publication (Kokai) No. 2005-275489, a controller 51 calculates a reference input such that an output Vex from an exhaust gas sensor converges to its target value Vex_cmd, and a
ΔΣ modulator 52 modulates the reference input with a ΔΣ modulation algorithm, whereby a fuel parameter Ufuel is calculated as a control input. This makes it possible to cause the output Vex from the exhaust gas sensor to converge to the target value Vex_cmd accurately, while compensating for response delay and variations of the engine and the catalytic device, thereby making it possible to ensure excellent reduction of exhaust emissions by the catalytic device.
According to the above-described conventional control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550, the air-fuel ratio is controlled based on the output VOUT from the oxygen concentration sensor disposed on the downstream side of the catalytic device. This can cause the following problems: In general, when the air-fuel ratio varies between cylinders of a multi-cylinder internal combustion engine, although exhaust gases emitted from the cylinders are mixed with each other on the downstream side of the collecting section of the exhaust passage, the degree of mixing between them is limited, and hence when the exhaust gases flow into the catalytic device, non-uniformity of the air-fuel ratio of the exhaust gases is sometimes caused in the catalytic device. Therefore, for example, even when the output VOUT from the oxygen concentration sensor has converged to the predetermined target value VOUT_TARGET, i.e. even when DVO2≈0 holds, a state is sometimes caused in which a half of the catalytic device is in a rich atmosphere, and the other half thereof is in a lean atmosphere. When the catalytic device is held in such a state for a long time period, the exhaust emission reduction rate of the whole catalytic device is reduced, resulting in increased exhaust emissions.
To solve the above-described problems of the control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550, it is envisaged to configure a control apparatus, as described below, by applying the control method disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-275489 to the control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550. The control apparatus is configured such that the above-mentioned operation amount Usl is modulated with the A ΔΣ modulation algorithm to thereby calculate a modulated operation amount, and the learned value FLAFBASE is added to the modulated operation amount, to thereby calculate the target air-fuel ratio KCMD. With this configuration, when DVO2≈0 holds, the target air-fuel ratio KCMD is calculated such that it repeatedly performs a high-frequency periodic fluctuating behavior with respect to the learned value FLAFBASE as the center. As a consequence, when the learned value FLAFBASE is equal to the optimum value of the target air-fuel ratio KCMD, which makes DVO2 appropriately equal to 0 (DVO2≈0), the air-fuel ratio of each cylinder performs the high-frequency periodic fluctuating behavior, which makes it possible to prevent the atmosphere of the catalytic device from being held in a non-uniform state for a long time period, thereby making it possible to ensure excellent reduction of exhaust emissions.
However, when the learned value FLAFBASE largely deviates from the optimum value of the above-mentioned target air-fuel ratio KCMD, the output VOUT from the oxygen concentration sensor largely deviates from the predetermined target value VOUT_TARGET, so that the fluctuating frequency of the target air-fuel ratio KCMD becomes lower. The present assignee has already confirmed that when the fluctuating frequency of the target air-fuel ratio KCMD becomes lower as described above, unless the catalytic device is degraded, it is possible to maintain an excellent exhaust emission reduction rate of the catalytic device, whereas if the catalytic device is degraded, the exhaust emission reduction rate becomes markedly lower (e.g. the publication of Japanese Patent No. 3880861 (Paragraph numbers [0202] to [0205], FIG. 38).
Recently, ethanol-mixed fuel is used as fuel for engines, and non-metal catalysts, such as Perovskite, low-precious metal catalysts, and so forth are used as catalysts for catalytic devices. In such cases, the optimum value of the target air-fuel ratio KCMD, which makes DVO2 appropriately equal to 0, is liable to change suddenly and largely with a change in the operating state of the engine (e.g. a change in the volume of exhaust gases). In contrast, as described above, the learned value FLAFBASE has a characteristic that the learning speed thereof is low, so that when the optimum value of the target air-fuel ratio KCMD is changed suddenly and largely with the change in the operating state of the engine, as described above, the learned value FLAFBASE is made liable to deviate from its optimum value largely, thereby making the fluctuating frequency of the target air-fuel ratio KCMD liable to be become markedly lower, which makes the above problem conspicuous.